This invention relates to the bonding of microelectronic chips to an electrically conducting bonding surface and, more particularly, to the bonding of semiconductor laser chips to their heat sinks.
A semiconductor laser is a tiny device typically measuring only about 250 .mu.m in width across the mirror facets and about 400 .mu.m in length along the optical resonator. The active region of the laser, where recombination of holes and electrons generates stimulated emission, is even smaller. In a typical stripe geometry, double heterostructure (DH) laser the active region may be only 3-5 .mu.m wide, 0.1-0.2 .mu.m thick and 400 .mu.m long. As a consequence, when a typical pumping current of, say, 100 mA, is applied to the laser, the current density in the active region may be of the order of 5000 A/cm.sup.2. Since the lasers are commonly operated continuous wave at or near room temperature, appreciable heating occurs. Excessive heating, of course, would damage the laser.
In order to prevent such damage, the removal of heat from the laser chip is facilitated by bonding the chip to a suitable heat sink, which is commonly made of a high thermal conductivity material (e.g., copper or diamond). However, the bonding operation must be performed carefully so as to avoid inducing in the laser chip excessive strain which, it has been found, is deleterious to laser reliability and lifetime. That is, the common laser substrate materials, GaAs and InP, are brittle and may crack under undue stress (e.g., 10.sup.9 dyn/cm.sup.2). Moreover, stress may generate defects in the active region or cause defects to migrate to the active region.
Thus, thermo-compression and ultrasonic chip bonding, widely used techniques in the other semiconductor fields, have been largely rejected in the semiconductor laser art in favor of soldering in the presence of a corrosive flux. In this method, a thick gold bonding pad is deposited on the ohmic contact on one side of the laser chip, and a thin gold layer is formed on a Cu stud (heat sink). A thick layer of a soft, ductile, low melting point metal (commonly indium) is evaporated onto the gold layer on the stud and is then placed in contact with the gold bonding pad. A small weight (e.g., 5 gm) is lowered onto the chip, and the stud temperature is raised to about 280.degree. C. in an atmosphere of forming gas in order to melt the indium and to bond the chip to the stud. However, the indium layer tends to oxidize before the bond is completed, and oxides in the bond are increase heat generation because of their high electrical resistivity and poor thermal conductivity. Also, because the melting point of the oxide is higher than that of the solder, the presence of a solid surface oxide prevents the solder from wetting at the bonding temperature. In order to insure ease and reproducibility in realizing these bonds, and to keep electrical conductance high, a liquid flux (e.g., ZnCl.sub.2 --NH.sub.4 Cl--H.sub.2) is used to dissolve the indium oxide and allow the liquid indium to flow and wet the gold on the laser chip.
This flux soldering procedure has several drawbacks, however. First, the danger exists of chlorine-containing residues becoming trapped within the bond. ZnCl.sub.2, for instance, transforms into ZnO, Zn(OH)Cl and HCl in the presence of small amounts of water and thus is a potential corrosive agent in devices operated in humid ambients. Second, the flux boils at the required bonding temperature of 260.degree.-280.degree. C. and becomes a carrier for liquid indium droplets which, if they hit the sides of the chip, can produce a short-circuit or cause interference with the light beam exiting the laser facet. Third, voids 20-100 .mu.m in size are created during bonding by bubbles of boiling flux entrapped inside the indium. These voids can drastically affect heat removal in the bond area because they represent barriers of practically infinite thermal resistance. If located directly under the active region, voids can raise the junction temperature considerably: theoretical calculations show that a 30 .mu.m void raises the junction temperature by 7.8.degree. C. and a 100 .mu.m void by 47.5.degree. C. Fourth, the bonds are frequently incompletely wetted; i.e., the actual bond area is smaller than the gold bonding pad. This shortcoming may be due to improper application of the flux which is applied manually and hence is very operator dependent. Fifth, the bonds frequently contain large amounts of Au--In intermetallic compounds (e.g., Au.sub.4 In, Au.sub.9 In, AuIn, and AuIn.sub.2) which can result in hard bonds. The absence of a pure indium layer after bonding or the presence of the harder intermetallic compounds allows thermal stresses (due to the difference in thermal expansion coefficients between the heat sink and the laser) as well as intrinsic stresses (induced by the presence of the intermetallics) to be transmitted to the active region of the laser.
Forming low-stress, void-free bonds is also of interest with other microelectronic chips, for example, LEDs which are bonded to a metallic header.